The continuing pursuit of more fuel efficient vehicles is requiring automatic transmissions to be more robust and energy efficient. There are a number of types of automatic transmission including stepped automatic transmissions, automated manual transmissions, continuously variable transmissions and dual clutch transmissions. Each type of automatic transmission offers some advantages over the others when used in vehicles; however, the ability to reduce size and weight while maintaining desired transmission characteristics provides a benefit regardless of type. In any automatic transmission where a fluid lubricated clutch is used (e.g. stepped automatic transmissions, continuously variable transmissions and dual clutch transmissions), increasing the friction level in the clutch has the desirable effect of increasing the level of torque that can be transferred through the clutch which, in turn, requires less surface area to transmit the same amount of torque.
However, different transmissions tend to have differing demands on a lubricant for optimal performance. For example, gear oil often requires good extreme pressure and load-bearing properties, generally low boundary friction and low thin film friction. On the other hand, a lubricant for continuously variable transmissions (CVT) tends to require low boundary friction and high thin film friction. Automatic transmissions with wet clutches, in contrast, usually require high boundary friction for an optimal torque transfer and low thin film friction to efficiently pump fluid through the transmission.
In many instances, a particular focus of automatic transmission fluids (ATF) is the minimization of shudder (that is, anti-shudder properties), which is believed to be a function of the change of friction coefficient with time (dμ/dt<0). Furthermore, shift characteristics of automatic transmissions are primarily dependent on the frictional characteristics of the ATF. The ATF fluid typically needs to have a high and stable frictional performance over the life of the fluid, good anti-shudder performance, and anti-wear characteristics. These characteristics are often a challenge to balance with the requirement that today's ATF lubricant compositions also need to maximize service intervals, or even better, avoid oil service during the lifetime of the equipment. This is referred to in the industry as a lifetime fill or “fill-for-life” fluid. Therefore, maintenance of the friction properties of an ATF over time, i.e. friction durability, may also be a desired property of the fluid.
Several prior efforts at improving the friction properties of ATF fluids have been attempted, but many involve the addition of or increased levels of lubricant components such as friction modifiers, oxidization modifiers, and/or viscosity modifiers. The addition of or increased levels of components escalates manufacturing complexity and increases product costs. Moreover, adding or increasing the level of friction modifies in the context of automatic transmission fluid may be undesired because it may compromise other important fluid characteristics. For instance, added or increased levels of friction and/or viscosity modifiers tend to drop static friction to levels resulting in unacceptable loss of torque transmission, which can be undesired in automatic transmissions, such as those including wet-clutch friction discs.